1. Technical Field
The present invention relates to hydrodynamic-pressure bearing devices employed in motors; in particular, to hydrodynamic-pressure bearing devices in which a shaft made with a thrust plate projecting radially outward with respect to the rotational axis, and a sleeve element surrounding the shaft rotate relative to one another via a lubricantxe2x80x94and to motors provided with the hydrodynamic-pressure bearing devices.
2. Description of Related Art
Hydrodynamic pressure bearings are employed a s bearing devices in such motors as hard-disk drive spindle motors in order to improve motor running characteristics and to achieve higher speed rotation an d low vibration (noise). FIG. 1 is a sectional view in which a hard-disk drive spindle motor equipped with a conventional hydrodynamic-pressure bearing device is illustrated.
The bearing device in FIG. 1 has: a shaft a having a thrust plate b at the lower end; a cylindrical sleeve member d in which a center hole or bore c is provided, with the bore wall surrounding the outer circumferential surface of the shaft a via a micro-gap; and an annular thrust groove e formed in the lower end of the sleeve d for accommodating the thrust plate b. The opening on the thrust-plate side end of the center hole c in the sleeve member d is covered by a cover member f.
Upper and lower radial bearing portions g and h in which lubricating oil is retained and having herringbone striations for generating hydrodynamic pressure are defined between the sleeve member d and the shaft a. A radial gap expansion i provided in between the upper and lower radial bearing portions g and h. The radial gap expansion i communicates with the exterior (the exterior of the sleeve member d within the spindle motor) via a vent hole j provided in the sleeve member d, which opens to the external atmosphere via a gap m formed between a rotor k and a bracket l.
Further, the radial gap expansion i communicates with a recess p formed in the lower end of the shaft a via a radial vent hole n and an axial vent hole o provided in the shaft a. An axial breathing hole q is provided in between the thrust plate b and the shaft a, and in the thrust plate b a breathing hole r that communicates an axially intermediate position of the breathing hole q with the outer circumferential surface of the thrust plate b is provided.
A pair of thrust bearing portions s in which lubricating oil is retained and having herringbone striations for generating hydrodynamic pressure are defined between the upper and lower faces of the thrust plate b, and the lower face of the thrust groove e and the upper face of the cover f.
Air bubbles mixed in the lubricating oil retained in the upper radial bearing portion g are, in the upper section thereof, released to the exterior from between the upper end of the sleeve member d and the shaft a, and respectively in the lower section thereof, released via the gap expansion i and the vent hole j. Air bubbles mixed in the lubricating oil retained in the lower radial bearing portion h are, in the upper section thereof, released to the exterior via the gap expansion i and the vent hole j, and respectively in the lower section thereof, released via the breathing hole q, the recess p, the axial vent hole o, the radial vent hole n, the gap expansion i and the vent hole j. Air bubbles mixed into the lubricating oil retained in the thrust bearing portions s are also released to the exterior via the breathing hole q, the recess p, the axial vent hole o, the radial vent hole n, the gap expansion i and the vent hole j. Accordingly, either axial end sections of the radial bearing portions and either radial end sections of the thrust bearing portionsxe2x80x94i.e., the sections in which the pressure due to the striations in each of the bearing portions is lowestxe2x80x94are respectively vented to the external atmosphere;
and releasing air bubbles mixed within the lubricating oil in these sections to the exterior. By virtue of the above-described structure, lubricating oil is prevented from overflowing/dispersing, due to temperature elevation and air-bubble expansion when the atmospheric pressure declines, to the exterior of the bearing device.
In hard-disk drive spindle motors, female threads t for fixing a disk clamp (not shown) are provided in the shaft a for clamping hard disk(s) on the motor with a screw (not shown), but in making the motors thinner, female threads t vie with the axial vent hole o, and the radial vent hole n, etc. for space within the shaft a, lowering the degree of freedom from a design aspect, and in some respects leading to difficulties in the manufacturing process. Moreover, conventional spindle motors make numerous vent holes and breathing holes as in the foregoing necessary, which has become a production cost-increase factor. In particular, the necessity of providing the breathing hole q and the breathing hole r as noted above in the thrust plate b makes costs high, and makes it difficult to form the shaft a and the thrust plate b integrally.
An object of the present invention is to provide a hydrodynamic-pressure bearing device manufacturable at low cost.
Another object of the invention is to provide a hydrodynamic-pressure bearing device in which air bubbles intermixed into the lubricant can be exhausted to the exterior of the bearing.
Yet a further object of the present invention is to provide a hydrodynamic-pressure bearing device of simple construction in which air bubbles intermixed into the lubricant can be exhausted to the exterior of the bearing, and that at the same time is manufacturable at low cost.
An another object of the invention is to provide a motor equipped with a hydrodynamic-pressure bearing device of simple construction in which air bubbles intermixed into the lubricant can be exhausted to the exterior of the bearing, and that at the same time is manufacturable at low cost.
A hydrodynamic-pressure bearing device of the present invention includes a shaft, a thrust plate projecting radially outward from the circumferential surface of the shaft, and a cylindrical sleeve element surrounding the shaft and thrust plate via a micro-gap. On the surfaces where the shaft and the sleeve element face each other radially, a pair of radial bearing portions that retain lubricant are configured so as to be axially separated from each other; and on the surfaces where the thrust plate and the sleeve element face each other axially, thrust bearing portions that retain lubricant are configured. One or the other of the pair of radial bearing portions retains lubricant continuously with the thrust bearing portions, in which radial bearing portion axially asymmetrical herringbone striations that generate hydrodynamic pressure acting toward the thrust bearing portion are formed as dynamic-pressure-generating grooves. Spiral striations that generate hydrodynamic pressure that acts heading radially inward are formed as dynamic-pressure-generating grooves in the thrust bearing portions. Lubricant is sustained on the outer periphery of the thrust plate, continuous with the thrust bearing portions. An annular recess is formed in between the pair of radial bearing portions so as to enlarge the radial dimension of the micro-gap, which is defined by the space between the outer circumferential surface of the shaft and the inner circumferential surface of the sleeve element by which it is radially opposed; air is retained in the annular recess, which separates the pair of radial bearing portions. A breathing hole that opens on the annular recess and the outer periphery of the thrust plate, and at the same time opens on the exterior of the bearing, is formed in the sleeve element. Air bubbles intermixed into the lubricant retained in the pair of radial bearing portions and the thrust bearing portions are exhausted or released to the exterior of the bearing through the breathing hole.
From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art.